Nickel-based alloy

ABSTRACT

The Ni-based alloy exhibits superior grain boundary corrosion resistance including C: 0.005 to 0.03 mass %, Si: 0.02 to 1 mass %, Mn: 0.02 to 1 mass %, P: not more than 0.03 mass %, S: not more than 0.005 mass %, Cr: 18 to 24 mass %, Mo: 8 to 10 mass %, Nb: 2.5 to 5.0 mass %, Al: 0.05 to 0.4 mass %, Ti: not more than 1 mass %, Fe: not more than 5 mass %, N: not more than 0.02 mass %, and Ni as a remainder and inevitable impurities. The C concentration range, the ratio of (Nb, Ti) C carbides to all carbides is not less than 90%, and the number of (Nb, Ti) C carbides satisfies the following formula: −30×T+37220=&lt;number of (Nb, Ti) C carbides (number/mm 2 )=&lt;−7.7×T 2 +15700×T−7866000 under a condition of 2000×% C+890=&lt;T(temperature ° C.)=&lt;1150.

TECHNICAL FIELD

The present invention relates a nickel-based alloy used for variouspurposes such as in chemical plants, natural gas pipes, and containers.

BACKGROUND ART

Ni-based alloys, in particular Ni—Cr—Mo—Nb alloys, are used in harshenvironments that are highly corrosive because such alloys have superiorcorrosion resistance. In this way, these alloys are used in harshenvironments in which there is the risk that Fe-based alloys will becorroded. Therefore, corrosion resistance at surfaces is particularlyimportant.

In order to apply corrosion resistance of Ni—Cr—Mo—Nb alloysufficiently, techniques concerning formation of passivation films areknown (for example, see Japanese Unexamined Patent ApplicationPublication No. 2015-183290). Since corrosion resistance is exhibited atthe surface, and surface conditions are particularly important. If thesurface is viewed microscopically, the surface is seen to be constructedof crystal grains. The surfaces of the crystal grains are sufficientlymaintained by a dense passivation film. However, there is a problem inthat the crystal grain boundary has less corrosion resistance. Thereason is that in Ni—Cr—Mo—Nb alloys, deposits containing Cr or Mo maybe formed at grain boundaries if conditions of heat treatment are notappropriate. Since the passivation film mainly containing Ni, Cr, Mo andO, which is effective for corrosion resistance, is difficult to beformed densely on the deposits, corrosion resistance may bedeteriorated. Corrosion resistance may be further deteriorated bysensitization. That is, in a neighborhood of the deposits containing Cror Mo, the Cr or Mo in the base material is dispersed to the deposits,and an absentee layer containing less of these elements is formed. SinceCr and Mo are effective for corrosion resistance, if the passivationfilm dissolves in a corrosive environment, corrosion occurs from thisabsentee layer of Cr and Mo, and thus, corrosion resistance is extremelydeteriorated.

In view of the above object, a technique in which Ni-based alloy havingno carbide is produced by performing solution heat treatment isdisclosed (for example, see Japanese Unexamined Patent ApplicationPublication No. Showa 57 (1982)-9861). Actually, according to thistechnique, the alloy has superior corrosion resistance at a step ofshipment from the factory. However, since Ni-based alloy is used beingprocessed as a pipeline, chemical plant, reaction vessel or the like,there may be a case in which the alloy is heat treated via theseprocessings or weldings. In that case, if an inappropriate heattreatment is performed, there may be a case in which deposits containingCr or Mo are formed at grain boundaries. Then, grain boundary corrosionresistance is deteriorated by the abovementioned mechanism, grainboundary corrosion is promoted, and in the worst case, a serious problemoccurs to the extent that corrosion penetrates the material. In thisway, it can be said to be a very important technique to prevent carbidescontaining Cr or Mo, which is effective element for corrosionresistance, from forming at grain boundaries.

A technique in which formation of carbides containing Cr and Mo isprevented in Ni—Cr—Mo—Nb alloy containing 11 to 20% of Mo is known (forexample, see Japanese Unexamined Patent Application Publication No.Heisei 7 (1995)-11404). That is, this is a technique to prevent carbidescontaining Cr and Mo from forming by depositing NbC at grain boundariesby performing aging heat treatment at 600 to 800° C. for 1 to 200 hours.However, it requires aging heat treatment at 600 to 800° C. and a longtime of 1 to 200 hours, and there is a problem in that it is notactually possible to perform the treatment after the pipeline, chemicalplant, reaction vessel or the like is completed. That is, the techniqueis a method that is impossible to employ industrially. In addition, thepublication describes nothing about size and density of NbC, and it isnot clear whether or not NbC is stabilized by this technique.

A Ni-based alloy exhibiting superior grain boundary breaking resistanceis proposed, which is developed by producing test pieces underconditions not depositing NbC, that is, solution heat treatment, and byevaluating with grain boundary corrosion resistance test while impartingstress (for example, see Japanese Unexamined Patent ApplicationPublication No. Heisei 5 (1993)-255787). As mentioned above, in acondition in which carbides are in a solid solution, inappropriate heattreatment after assembling pipelines, chemical plants, reaction vesselsor the like may cause formation of deposits containing Cr or Mo at grainboundaries, and thus, the technique is not practical.

Furthermore, a technique in which solution heat treatment is performedat 1000 to 1100° C. and rapid cooling is performed at not less than 200°C./sec so as to solid-solve carbide (for example, see JapaneseUnexamined Patent Application Publication No. Heisei 5 (1993)-140707).Corrosion resistance can be reliably obtained if these conditions can berealized. However, it is not possible to actually perform the heattreatment and rapid cooling after the pipeline, chemical plant, reactionvessel or the like is completed, and thus, the technique is notpractical.

SUMMARY OF THE INVENTION

In view of the abovementioned conventional techniques, an object of thepresent invention is to make clear what effects the amount of C impartson deposition behavior of carbides in order to control depositioncontaining Cr or Mo in Ni-based alloys and to provide a Ni-based alloyexhibiting superior grain boundary corrosion resistance.

The inventors performed research in order to solve the above problems.They evaluated products that were actually produced by using realequipment. That is, a slab produced by a continuous casting apparatuswas hot rolled to obtain a hot rolled plate having a thickness of 6 mm,and the plate was cold rolled to produce a cold rolled plate having athickness of 4 mm A test piece having a size of 20×25 mm was taken fromthe cold rolled plate. The present invention was completed according toa correlative relationship of NbC ratio, M6C (M is mainly Mo, Ni, Cr, orSi) ratio, M23C6 (M is mainly Cr, Mo, or Fe) ratio, NbC density and sizefactor and results of a grain boundary corrosion resistance test. Thatis, they found that grain boundary corrosion resistance can be highlymaintained by restraining deposition of M6C and M23C6 and by effectivelydepositing NbC. This invention enables more accurate control byrevealing a quantitative relationship of C concentration and temperatureby analyzing an equilibrium diagram of a multicomponent type alloy ofNi—Cr—Mo—Nb base alloy in detail.

In particular, in the alloy, not only is the addition effect of Nbextremely important in increasing strength, but Nb is also extremelyimportant for preventing sensitization conditions which deteriorategrain boundary corrosion resistance. The reason is that C combines withNb in order to keep Cr and Mo (these are important elements to keepgrain boundary corrosion resistance in a good condition) in a solidsolution condition, and NbC is formed. The present invention wasdeveloped based on the above knowledge.

That is, the Ni-based alloy of the present invention includes C: 0.005to 0.03 mass %, Si: 0.02 to 1 mass %, Mn: 0.02 to 1 mass %, P: not morethan 0.03 mass %, S: not more than 0.005 mass %, Cr: 18 to 24 mass %,Mo: 8 to 10 mass %, Nb: 2.5 to 5.0 mass %, Al: 0.05 to 0.4 mass %, Ti:not more than 1 mass %, Fe: not more than 5 mass %, N: not more than0.02 mass %, with Ni as the remainder and inevitable impurities, whereinwithin the C concentration range, the ratio of (Nb, Ti) C carbides toall carbides is not less than 90%, and the number of (Nb, Ti) C carbidessatisfy following formula:

−30×T+37220=<number of (Nb,Ti)C carbides (number/mm²)=<−7.7×T²+15700×T−7866000 under a condition of 2000×% C+890=<T(temperature °C.)=<1150.

It is desirable that PRE value=Cr %+3.3Mo %+16N % is not less than 50and the size of the (Nb, Ti) C carbides be in a range of 0.03 to 3 μm inthe Ni-based alloy of the present invention.

It is desirable that corrosion rate in ASTM G28 Method A test be lessthan 1.5 mm/y in the Ni-based alloy of the present invention.

It is desirable that corrosion rate in ASTM G28 Method A test after heattreatment at 500 to 800° C. for 1 to 20 h be less than 1.5 mm/y in theNi-based alloy of the present invention.

It is desirable that precipitation of carbides containing more than 50%of Mo or Cr is suppressed to less than 10% of all carbides, bydispersing the (Nb, Ti) C carbides in hot rolling at temperatures of10⁴×C %+950 to 2000×% C+890° C. in the Ni-based alloy of the presentinvention.

Furthermore, the Ni-based alloy of the present invention includes C:0.005 to 0.03 mass %, Si: 0.02 to 1 mass %, Mn: 0.02 to 1 mass %, P: notmore than 0.03 mass %, S: not more than 0.005 mass %, Cr: 18 to 24 mass%, Mo: 8 to 10 mass %, Nb: 2.5 to 5.0 mass %, Al: 0.05 to 0.4 mass %,Ti: not more than 1 mass %, Fe: not more than 5 mass %, N: not more than0.02 mass %, with Ni as the remainder and inevitable impurities, whereinwithin the C concentration range, the ratio of (Nb, Ti) C carbides toall carbides is not less than 90%, and the number of (Nb, Ti) C carbidesis 6000 to 100000 (number/mm²).

It is desirable that N: 0.002 to 0.02 mass % in the Ni-based alloy ofthe present invention.

It is desirable that the size of the (Nb, Ti) C carbides be 0.03 to 3 μmin the Ni-based alloy of the present invention.

It is desirable that corrosion rate in ASTM G28 Method A test be lessthan 1.5 mm/y in the Ni-based alloy of the present invention.

It is desirable that corrosion rate in ASTM G28 Method A test after heattreatment at 500 to 800° C. for 1 to 20 h be less than 1.5 mm/y in theNi-based alloy of the present invention.

Deposition of carbides of Cr or Mo can be restrained by forming (Nb, Ti)C carbides. Since grain boundary corrosion resistance can be maintainedin an appropriate condition by that, grain boundary corrosion resistanceis prevented from deteriorating even by the heat treatment performed atan actual site after shipment of the alloy, and thus, a material for usein extremely severe environments can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing an equilibrium diagram in the Ni-based alloyof the present invention, and showing a relationship between temperatureand carbon content (mass %).

FIG. 2 is a graph showing a relationship between the number of (Nb, Ti)C carbides (number/mm²) and heat treatment temperature.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, reasons for limitations of composition ranges of thepresent invention are explained. It should be noted that all “%” means“mass %”.

C: 0.005 to 0.03%

C is an effective element for maintaining strength of an alloy.Therefore, at least 0.005% is necessary. However, this combines Cr or Moand deposits carbides in a heat influenced part or the like in a heattreatment process or welding. Since Cr and Mo are effective elements formaintaining corrosion resistance, an absentee layer is generated arounddeposits, and grain boundary corrosion resistance is deteriorated.Therefore, C is specified as not more than 0.03%. Therefore, C isspecified in the range of 0.005 to 0.03%. The content is desirably 0.007to 0.028%, more desirably 0.01 to 0.02%, and further more desirably0.011 to 0.018%.

Si: 0.02 to 1%

Si is an effective element for deoxidation, and at least 0.02% isnecessary. However, since it also promotes formation of M6C and M23C6and deteriorates grain boundary corrosion resistance, Si should bereduced to not more than 1%. Therefore, Si is specified in the range of0.02 to 1%.

Mn: 0.02 to 1%

Mn is an effective element for deoxidation, and at least 0.02% isnecessary. However, since it easily promotes formation of MnS anddeteriorates pitting corrosion resistance at over 1%, Mn is specified inthe range of 0.02 to 1%.

P: not more than 0.03%

Since P is a harmful element for hot workability, and it should beremoved as much as possible. Therefore, P is specified at not more than0.03%.

S: not more than 0.005%

Since S is also a harmful element for hot workability as is P, it shouldbe removed as much as possible. Therefore, S is specified at not morethan 0.005%.

Cr: 18 to 24%

Cr is an important element to form a passivation film and maintaincorrosion resistance. Therefore, it is necessary that a base materialcontain Cr at not less than 18%. However, excess content causes M23C6 (Mis mainly Cr, Mo, and Fe) to deposit easily. Since this tendency isnoticeable and corrosion resistance is deteriorated if the content ismore than 24%, Cr is specified in the range of 18 to 24%. The content isdesirably 20 to 23% and is more desirably 21 to 22.8%.

Mo: 8 to 10%

Mo is an important element to form a passivation film and maintaincorrosion resistance. Therefore, it is necessary that a base materialcontain Mo at not less than 8%. However, excess content causes M6C (M ismainly Mo, Ni, Cr, and Si) to deposit easily and strength is increased,thereby worsening workability, and the content is specified in the rangeof 8 to 10%. The content is desirably 8.1 to 9.0% and is more desirably8.2 to 8.7%.

Nb: 2.5 to 5.0%

Nb is an element improving strength. Furthermore, since it combines withcarbon so as to form NbC, it exhibits important effects in which Mo andCr are prevented from combining with carbon. Therefore, it also has afunction to increase grain boundary corrosion resistance. However, atemperature at which ductility is exhibited is reduced and hotprocessing is impossible if not less than 5% is contained. Therefore,the content is specified in the range of 2.5 to 5.0%. The content isdesirably 3 to 4.8% and is more desirably 3.5 to 4.5%.

Al: 0.05 to 0.4%

Al is an important element for deoxidation and desulfuration. At least0.05% is necessary in order to perform deoxidation and desulfuration andsatisfy a S content of the present invention of not more than 0.005%.However, there is a risk of formation of alumina clusters if more than0.4% is added. Therefore, the content is specified in the range of 0.05to 0.4%. The content is desirably 0.1 to 0.35% and more desirably 0.15to 0.33%.

Ti: Not more than 1%

Ti is an effective element to improve strength. Furthermore, Ti combineswith carbon so as to form TiC in a manner similar to Nb, and it preventsCr and Mo from forming carbides. Therefore, it also has a function toincrease grain boundary corrosion resistance, and the content isspecified not more than 1%.

Fe: Not more than 5%

Fe is often added to reduce production cost; however, the content is setto be not more than 5% since high Fe concentration in a passivation filmmay result in decrease of corrosion resistance. The content is desirablynot more than 4.8% and is more desirably not more than 4.7%.

N: Not more than 0.02%

N should be reduced as much as possible because it forms TiN whichclusters and causes surface defects. Therefore, the content is specifiedas being not more than 0.02%. On the other hand, addition of a minimumamount is desirable in order to exhibit strength and corrosionresistance, and it is desirable to add not less than 0.002%. The contentis more desirably 0.002 to 0.015%. It should be noted that Nconcentration was accurately controlled by nitrogen gas uptake oraddition of ferrochrome nitride during AOD or VOD.

Basically, the alloy of the present invention is a Ni-based alloy. Thereason is as follows. Since Ni is a noble metal, corrosion thereof issuperior to that of Fe. Since unlike a case in which Fe generateshydroxide Fe(OH)₂, Ni does not generate hydroxide in a passivation film,the passivation film is dense and has high protecting effect. Inaddition, since the amount of an alloy element which can solid-solve isgreater in a Ni-based alloy than in an Fe based alloy, the Ni-basedalloy can contain more of an element that increases corrosion resistancesuch as Cr or Mo. Therefore, in order to form a protecting film havingsuperior corrosion resistance on a surface of a base material, aNi-based alloy must be chosen. In addition, an inevitable impurity inthe present invention is Cu, Co, W, Ta, V, B and H.

The reason that the ratio of (Nb, Ti)C carbides to all carbides must benot less than 90% in the above C concentration range (C: 0.005 to 0.03%)is explained. This is because a degree of corrosion of less than 1.5mm/y in the ASTM G28 Method A test cannot be achieved unless the ratioof deposition of M6C and M23C6 is reduced to less than 10%.

The basis for the number of (Nb, Ti) C carbides satisfying the formula:

−30×T+37220=<number of (Nb,Ti)C carbides (number/mm²)=<−7.7×T²+15700×T−7866000

under a condition of 2000×% C+890° C.=<T(temperature ° C.)=<1150 wasexperimentally verified and it was concluded from consistency with theequilibrium diagram. If the condition is satisfied, the ratio of (Nb,Ti)C carbides becomes not less than 90% and the corrosion degree lessthan 1.5 mm/y can be achieved in the ASTM G28 Method A test.Furthermore, in a range of temperature 30° C. higher than a boundary of10⁴×C %+950, since M6C and M23C6 is solid-solved and NbC partiallyremained, a corrosion degree of less than 1.5 mm/y could be achieved inthe ASTM G28 Method A test after stress releasing annealing.

It is very important to measure a number distribution of theabovementioned (Nb, Ti)C carbides accurately. It is necessary that afterheat treatment is performed at the temperature, first, cooling berapidly performed, and a condition at the temperature be maintained.Therefore, cooling is performed at not less than 50° C./sec. Acold-rolled plate having a thickness of 4 mm produced as mentioned abovewas cut to a size of 10×10 mm Wet polishing was performed on a crosssection perpendicular to a rolled direction, and furthermore,electrolytic polishing was performed, and the polished surface wasobserved by a FE-SEM so as to measure the number distribution.Composition of the carbides was identified by quantitative analysis.

The reason that PRE value=Cr %+3.3Mo %+16N % need not be less than 50 isexplained. In order to form a dense passivation film on the surface, thePRE is defined as being not less than 50. It should be noted thatalthough this is not particularly limited, it is desirable to let standfor four days in air or that a passivation treatment be performed inorder to obtain a dense passivation film.

The reason that size of the (Nb, Ti) C carbides must be in a range of0.03 to 3 μm is explained. If it is dispersed finer than 0.03 μm,crystal grains may be finer by a pinning effect, and cold workabilitymay be deteriorated. On the other hand, if it is larger than 3 μm, sincea dense passivation film is not formed in the deposition, this locationmay be an origin of corrosion, and there is a risk of crevice corrosionoccurring. Therefore, the range is specified in the range of 0.03 to 3μm. The range is more desirably 0.1 to 2 μm.

By satisfying the range of the invention, the corrosion degree can beless than 1.5 mm/y in the ASTM G28 Method A test. According to thecircumstances, in order to release stress which was induced duringprocessing and welding, there is a case in which the alloy is heattreated at 500 to 800° C. for 1 to 20 hours. By satisfying the range ofthe invention, the corrosion degree can be less than 1.5 mm/y in theASTM G28 Method A test. It is desirably less than 1.3 mm/y, moredesirably 1.2 mm/y, and further more desirably less than 1 mm/y.

If hot rolling is performed at a temperature 10⁴×C %+950 to 2000×C%+890° C., as mentioned above, since (Nb, Ti)C carbides can be moreeffectively deposited, temperature of the hot rolling is set in therange of 10⁴×C %+950 to 2000×C %+890° C.

Examples

Raw materials such as scraps, Ni, Cr, Mo and the like were melted in anelectric furnace, and decarburization was performed by at least one ofblowing oxygen in AOD (Argon Oxygen Decarburization) and VOD (VacuumOxygen Decarburization). Then, Cr reduction was performed by adding Aland lime, lime and fluorite were further added so as to formCaO—SiO₂—Al₂O₃—MgO—F type slag on the molten alloy, and deoxidation anddesulfuration were performed. SiO₂ concentration in the slag wascontrolled to be not more than 10%. The molten alloy refined in this waywas cast by a continuous casting apparatus so as to obtain a slab.

After that, the slab was hot rolled by a Steckel Mill, and it was thencold rolled so as to obtain a cold rolled plate having a thickness of 4mm. The chemical compositions of the alloys produced are shown in Table1, and the measurement conditions and evaluation results are shown inTable 2. In Tables 1 and 2, a value in brackets is out of the range ofthe present invention. Hereinafter, the evaluation method is shown.

(1) Analysis was performed by fluorescent X-ray analysis. However, C andS were analyzed by a combustion weight method and O was analyzed by aninert gas impulse melting IR absorption method.(2) Temperature of hot rolling was measured by a radiation thermometerafter finish rolling by a Steckel Mill and before water cooling.(3) Number distribution of (Nb, Ti)C: It is very important to accuratelymeasure the number distribution of (Nb, Ti)C carbides. It was necessarythat after heat treatment was performed at the temperature first,cooling was rapidly performed, and a condition of at the temperature wasmaintained. Therefore, cooling was performed at not less than 50°C./sec. A cold-rolled plate having a thickness of 4 mm produced asmentioned above was cut to a size of 10×10 mm Mirror polishing wasperformed on a cross section parallel to a rolled direction, and thepolished surface was observed by a FE-SEM so as to measure the numberdistribution. It should be noted that an area measured was 1 mm².(4) Composition of the carbides was identified by quantitative analysisby using EDS.(5) Size of (Nb, Ti)C was measured by FE-SEM as mentioned above. Itshould be noted that a size shown in Table 2 is an average valuerepresenting measured values.(6) Evaluation of grain boundary corrosion resistance: an annualcorrosion depth (mm/y) was evaluated by the ASTM G28 Method A test.(7) SR means stress release annealing, and heat treatment at 600° C. for5 hours was performed. This heat treatment replicates a heat treatmentthat is performed at the customer end after shipment of an alloy and isa cause of deterioration of grain boundary corrosion resistance.

TABLE 1 Chemical composition (mass %) remainder Ni No. C Si Mn P S Cr MoNb Al Ti N Fe Examples 1 0.006 0.25 0.25 0.014 0.0002 22.51 8.51 3.310.24 0.25 0.009 4.61 2 0.012 0.23 0.18 0.016 0.0003 23.67 8.35 3.25 0.390 0.012 2.53 3 0.018 0.22 0.85 0.015 0.0003 20.31 9.24 3.89 0.25 0.370.015 4.52 4 0.008 0.15 0.38 0.019 0.0032 23.56 8.04 2.68 0.08 0.170.008 1.85 5 0.019 0.42 0.45 0.011 0.0001 22.14 8.89 3.51 0.38 0.560.006 2.04 6 0.017 0.07 0.23 0.022 0.0005 21.85 8.45 4.23 0.28 0.810.016 0.91 7 0.013 0.18 0.42 0.019 0.0005 20.12 8.32 3.21 0.35 0.330.012 3.22 Comparative 8 (0.004) 0.25 0.29 0.018 0.0003 (17.45) 8.222.98 0.38 0.24 (0.001) 4.61 Example 9 0.012 0.61 0.41 0.02 0.0002 20.529.12 3.78 0.35 0.45 0.012 2.53 10 (0.032) 0.03 0.41 0.022 0.0015 22.85(7.23) 3.89 0.03 0.24 0.013 4.52 11 0.005 0.24 0.32 0.019 0.0002 22.318.59 2.75 0.42 0.25 (0.024) 1.85 12 (0.003) 0.24 0.33 0.018 0.0002 22.518.21 3.51 0.48 0.48 0.012 2.04 13 (0.024) (1.45) 0.45 0.017 0.0001 22.39(11.5) 4.23 0.38 0.22 0.012 3.11 14 (0.001) 0.24 0.35 0.018 0.0005 21.858.24 3.21 0.44 0.18 0.023 3.22

TABLE 2 (Nb,Ti)C deposition condition Hot rolling condition −7.7 ×T{circumflex over ( )}2 + Boundary {circle around (2)} Boundary {circlearound (1)} −30 × T + 15700 × T − Hot rolling 2000 × % C + 10{circumflexover ( )}4 × Heat 37220 7866000 temperature 890 % C + 950 treatmentnumber/ number/ No. ° C. ° C. ° C. ° C. mm² mm² Examples 1 920 902 10101140 3020 25080 2 980 914 1070 1145 2870 15608 3 940 926 1130 1150 27205750 4 920 906 1030  950 8720 99750 5 980 928 1140  940 9020 88280 61060  924 1120 1100 4220 87000 7 1030  916 1080 1000 7220 134000Comparative 8 (1060)  898 990 1100 4220 87000 Example 9 (1100)  914 1070(1220) 620 — 10 (920) 954 1270 1100 4220 87000 11 (1070)  900 1000 (780)13820 — 12 (700) 896 980 (800) 13220 — 13 1100  938 1190 (900) 1022027000 14 910 892 960 (750) 14720 — Test I Test II Results ASTM G28 ASTMG28 (Nb,Ti)C Method Method density (Nb,Ti)C A test A test after (Nb,Ti)Cnumber/ PRE size result SR result No. ratio % mm² value μm mm/y mm/yExamples 1 98 22.000 50.7 0.1 0.45 1.23 2 92 14.000 51.4 0.4 0.43 1.14 397  6.800 51.0 0.9 0.51 1.44 4 92 98.000 50.2 0.09 0.44 0.78 5 94 80.00051.6 0.5 0.48 0.69 6 100  73.000 50.0 2.2 0.32 0.56 7 98 61.000 47.8 0.80.39 0.67 Comparative 8 95 (100)    44.8 1.2 1.86 2.4 Example 9  (0)(0)   50.8 5.4 1.67 2.3 10 (30)  (3.800) 46.9 6.8 1.91 2.5 11  (5) (0)  51.0 0.02 1.76 2.1 12 (10) (10)    49.8 0.02 2.12 2.7 13  (3) (20)   60.5 0.01 1.82 3.56 14  (0) (0)   49.4 — 2.76 3.67

Effects of the present invention are clear by the Examples in Table 2.In addition, FIG. 1 shows an equilibrium diagram from the results of theresearch. In the equilibrium diagram in FIG. 1, 10⁴×C %+950 shown in theclaim 5 is a boundary 1 and 2000×% C+890 is a boundary 2. In addition,in Table 2, the test I means the ASTM G28 Method A test, and the test IImeans the ASTM G28 Method A test after stress releasing.

The hot rolling temperature was between the boundaries 1 and 2 shown inFIG. 1 in a case of Nos. 1 to 3 which are the inventive examples, and(Nb, Ti)C was deposited in the range; however, since annealingtemperature is in a range between the boundary 1 and 1150° C., theresult of test I satisfy less than 1.5 mm/y, which was good.Furthermore, the annealing temperature of Nos. 1 and 3 in heat treatmentwas within a temperature range 30° C. higher than the boundary (1).Therefore, although it is a range in which C is easily solid-solved,since (Nb, Ti)C remains, the result of the test II was also good. Itshould be noted that the value of the corrosion resistance was withinthe range of the invention; however, it was relatively high.

The hot rolling temperature was between the boundaries 1 and 2 and heattreatment thereafter was also appropriate in Nos. 4 to 7, and theresults of the tests I and II were both good.

Since the amount of C was lower than the lower limit value of 0.005% inNo. 8, strength was low. Furthermore, since Cr and N amounts were lowerthan the lower limitation values of 18% and 0.001% respectively,corrosion resistance was low and the PRE value was not more than 50.Furthermore, since hot rolling finishing temperature and annealingtemperature were not less than the boundary 1 and not more than 1150°C., C was in a condition of a solid solution. Therefore, since M6C andM23C6 were solid-solved, although (Nb, Ti)C was 95%, the number wasless, which was 100 number/mm². Therefore, the results were both above1.5 mm/y in the tests I and II.

Since hot rolling finishing temperature and annealing temperature werenot less than the boundary 1 and further annealing temperature was above1150° C. in No. 9, (Nb, Ti)C was completely in solid solution conditionand was not deposited. Therefore, the results were both above 1.5 mm/yin the tests I and II.

The amount of C was 0.032%, which is high, and a boundary 1 was 1270° C.and a boundary 2 was 954° C. in No. 10. Since hot rolling finishingtemperature was 920° C., which is lower than the boundary 2, M6C wasdeposited after hot rolling. Since the annealing temperature was 1100°C., which was between the boundaries 1 and 2, (Nb, Ti)C was easilydeposited. Therefore, the ratio of (Nb, Ti)C was about 30%, and thenumber of (Nb, Ti)C was 5,000 number/mm², which is small. As a result,the results were both above 1.5 mm/y in the tests I and II.

The amount of C was 0.005%, which is the lower limit value, a boundary 1was 1000° C., and a boundary 2 was 900° C. in No. 11. Since hot rollingfinishing temperature was 1070° C., which is higher than the boundary 1,the amount of C after hot rolling was in a solid solution condition. Onthe other hand, since the annealing temperature was 780° C., M6C andM23C6 were deposited. Therefore, the ratio of (Nb, Ti)C was 5%, and thenumber of (Nb, Ti)C was 200 number/mm², which is small. As a result, theresults were both above 1.5 mm/y in the tests I and II. In addition, theN value was 0.024%, which is above the upper limit value, and TiNclusters were generated and nozzle blocking occurred in a continuouscasting.

The amount of C was lower than 0.005% which is the lower limit value,strength was low, a boundary 1 was 980° C., and a boundary 2 was 896° C.in No. 12. Since hot rolling finishing temperature and annealingtemperature were both lower than the boundary 2, M6C and M23C6 weredeposited. The ratio of (Nb, Ti)C was 5%, and number of (Nb, Ti)C was200 number/mm², which is small. Furthermore, PRE was also not more than50, and as a result, the results were both above 1.5 mm/y in the tests Iand II.

The amounts of C, Si, and Mo were higher than each upper limit value,and the composition easily gave rise to deposition of large amounts ofM6C in No. 13. A boundary 1 was 1190° C., a boundary 2 was 938° C., hotrolling temperature was 1100° C. and annealing temperature was 900° C.MC was deposited after hot rolling and M6C and M23C6 were depositedafter annealing. Since large amounts of M6C were deposited by thecomposition and annealing conditions, the ratio of (Nb, Ti)C was low,and the number of (Nb, Ti)C was 20 number/mm², which is small. As aresult, the results were both above 1.5 mm/y in the tests I and II.

Since the amount of C was not more than the lower limit value, which waslowest in the Examples, strength was low in No. 14. A boundary 1 was960° C., a boundary 2 was 892° C., hot rolling finishing temperature wasnot less than the boundary 2 within 30° C., and annealing temperaturewas not more than the boundary 2. (Nb, Ti)C was not deposited. As aresult, the results were both above 1.5 mm/y in the tests I and II.

According to the promising present invention, Ni-based alloy having highgrain boundary corrosion resistance can be produced, which can restraindeterioration of grain boundary corrosion resistance even by a heattreatment performed at an actual site after shipment of an alloy andwhich can be used for a long time in extremely severe grain boundarycorrosion environments.

What is claimed is:
 1. A Ni-based alloy comprising: C: 0.005 to 0.03mass %, Si: 0.02 to 1 mass %, Mn: 0.02 to 1 mass %, P: not more than0.03 mass %, S: not more than 0.005 mass %, Cr: 18 to 24 mass %, Mo: 8to 10 mass %, Nb: 2.5 to 5.0 mass %, Al: 0.05 to 0.4 mass %, Ti: notmore than 1 mass %, Fe: not more than 5 mass %, N: not more than 0.02mass %, Ni as a remainder and inevitable impurities, wherein within theC concentration range, the ratio of (Nb, Ti) C carbides to all carbidesis not less than 90%, and the number of (Nb, Ti) C carbides satisfiesfollowing formula:−30×T+37220=<number of (Nb,Ti)C carbides (number/mm²)=<−7.7×T²+15700×T−7866000 under the conditions of 2000×% C+890°C.=<T(temperature ° C.)=<1150.
 2. The Ni-based alloy according to claim1, wherein PRE value=Cr %+3.3Mo %+16N % is not less than 50 and the sizeof (Nb, Ti) C carbides is in a range of 0.03 to 3 μm.
 3. The Ni-basedalloy according to claim 1, wherein corrosion rate in ASTM G28 Method Atest is less than 1.5 mm/y.
 4. The Ni-based alloy according to claim 1,wherein corrosion rate in ASTM G28 Method A test after heat treatment at500 to 800° C. for 1 to 20 his less than 1.5 mm/y.
 5. The Ni-based alloyaccording to claim 1, wherein precipitation of carbides containing morethan 50% of Mo and Cr is suppressed to less than 10% of all carbides, bydispersing the (Nb, Ti) C carbides in hot rolling at temperatures of10⁴× C %+950 to 2000× % C+890° C.
 6. A Ni-based alloy comprising: C:0.005 to 0.03 mass %, Si: 0.02 to 1 mass %, Mn: 0.02 to 1 mass %, P: notmore than 0.03 mass %, S: not more than 0.005 mass %, Cr: 18 to 24 mass%, Mo: 8 to 10 mass %, Nb: 2.5 to 5.0 mass %, Al: 0.05 to 0.4 mass %,Ti: not more than 1 mass %, Fe: not more than 5 mass %, N: not more than0.02 mass %, Ni as a remainder and inevitable impurities, wherein withinthe C concentration range, the ratio of (Nb, Ti) C carbides to allcarbides is not less than 90%, and the number of (Nb, Ti) C carbides is6000 to 100000 (number/mm²).
 7. The Ni-based alloy according to claim 6,wherein N: 0.002 to 0.02 mass %
 8. The Ni-based alloy according to claim6, wherein the size of (Nb, Ti) C carbides is 0.03 to 3 μm.
 9. TheNi-based alloy according to claim 6, wherein corrosion rate in ASTM G28Method A test is less than 1.5 mm/y.
 10. The Ni-based alloy according toclaim 6, wherein corrosion rate in ASTM G28 Method A test after heattreatment at 500 to 800° C. for 1 to 20 his less than 1.5 mm/y.